Within the context of petroleum drilling and completion systems, existing methods to provide hydraulic isolation (sealing) between portions of a wellbore or wellbore annulus, whether cased or open, may be broadly divided into two types of seal element: 1) bulk expansion (compression-set); and 2) inflatable. Devices employing either of these element methods are commonly referred to as either bridge plugs or packers, depending respectively, on whether full cross-sectional or annular closure is ultimately required. Since closure of an annular space with respect to the device is always required, the term “packer” is employed here to refer generally to all such devices.
Both bulk expansion and inflatable seal elements must provide sufficient annular clearance firstly to permit insertion into the wellbore to the desired depth or location, and secondly a means to subsequently close this annular clearance to affect an adequate degree of sealing against a pressure differential. It is often also desirable to retract or remove these devices without milling or machining.
Devices relying on bulk expansion of the seal element typically employ largely incompressible but highly deformable materials such as elastomers as the sealing element or element “stack”. The seal is generally cylindrically or toroidally shaped, and is carried on an inner mandrel. U.S. Pat. Nos. 5,819,846 and 4,573,537 are two examples of such devices using an elastomer and ductile metal (non-elastomeric), respectively, for the deformable seal element material. In these cases, the seal is formed by imposing axial compressive displacement of the element which causes the material to incompressibly expand radially (inward or outward or both) to close off either annular region. After confinement is achieved, sufficient pre-stress is applied to promote sealing.
The amount of annular expansion and sealing achievable with elastomers is dependent on several variables but is generally limited by the extrusion gap allowed by the running clearance. The size of annular gap sealable with ductile metals is similarly limited, although for slightly different reasons: since the deformation is largely irreversible, the size presents a further impediment to retrieval. For either elastomers or ductile metals, practically achievable axial-seal lengths are also short—in the order of a few inches. Therefore, sealing on rough surfaces is not readily achievable. This limitation to sealing small clearances with relatively short seal lengths and limited conformability even for elastomers tends to preclude using the method for sealing against most open-bore-hole surfaces. Furthermore, this style of device usually also provides a means to retract axial load, e.g., slips, separate from the sealing element.
Such axial loads arise from pressure differentials acting on the sealed area, plus loads transmitted by attached or contacting members and typically exceed either the frictional or strength capacity of the seal material. This is especially true as the sealed area (hole diameter) is increased. Managing the setting and possible release of the associated anchoring systems adds considerable complexity to these devices, along with increased cost and reliability issues. Similarly, the degree of complexity, cost and uncertainty is further increased where the application requires axial-load reversal that arises when the pressure differential may be in either direction. Both the sealing and mechanical-retaining hardware tend to require significant annular space. Therefore, the maximum internal-bore diameter is significantly smaller than the setting diameter.
Devices relying on inflation of the “membrane” seal element employ a generally cylindrical sealing element (visualize a hose), capable of expanding radially outward when pressured the inside with a fluid. The sealing element is carried on a mandrel with the end-closure means to contain pressure and to accommodate whatever axial displacement is required during inflation. The sealing element in these devices is typically of composite construction where an elastomer is reinforced by stiffer materials such as fibre strands, wire, cable, or metal strips (also commonly referred to as “slats”).
U.S. Pat. No. 4,923,007 is an example of a device employing axially aligned overlapping metal strips. Pressure containment by these elements relies largely on membrane action where the sealing element may be considerably longer and more conformable than in bulk expansion devices. Inflation packers are therefore most commonly employed for sealing against the open-bore-hole wall. The inflation material may be a gas, liquid or “setting” liquid such as cement slurry. Where the inflation material stays fluid, pressure must be continuously maintained to affect a seal. If the device develops a leak after inflating, the sealing function will be lost. To circumvent this weakness, a setting liquid such as cement is used. Therefore, pressure need only be maintained until sufficient strength is reached. However, the device then becomes much more difficult to remove since it cannot be retracted through reverse flow of the inflation fluid. Typically, the device can only be removed by machining and milling.
As with the bulk expansion method, the membrane strength of inflatable packers significantly limits the ability to react axial load and the annular space requirements of membrane end seals and mandrel can be quite large. Therefore, inflatable packer elements tend to suffer from the same limited axial load and through bore capacities as bulk expansion packer elements.